The present invention relates to the power amplification of a composite signal, and more specifically to its linear amplification.
The term "composite signal" should be understood to mean a signal formed by a first signal and a second signal that are independent of each other, it being understood that each of the two independent signals may themselves be a composite signals.
There is a known way, for example in television, of carrying out a linear power amplification of a composite signal formed by the image signal and the sound signal. The present invention involves single-channel amplification as opposed to amplification with separate channels where, for example in television, the image and sound signals are amplified by two distinct channels.
To carry out a single-channel linear amplification, there are known ways of using an output amplifier up to its saturation zone but not including the saturation zone, by associating a complementary amplifier with this output amplifier. The complementary amplifier is placed in series before the output amplifier and its transfer characteristic is chosen so as to be complementary to that of the output amplifier, i.e. so that the transfer characteristic of these two amplifiers is a straight line between two points known as saturation points respectively representing the limits of linear amplification of a positive signal and a negative signal. Beyond these points, the gain of the set or unit formed by the two amplifiers is zero or almost zero.
Taking the signal with the greatest amplitude as a first independent signal, the second signal has the effect of shifting the first signal on each side of the position that it would occupy if it were alone. This means that, giving the value 1 to the peak amplitude of the first signal, the composite signal oscillates between two limit values 1+D and -1-D, where D is the maximum value of shift.
Known linear amplification devices comprise the following in series: the complementary amplifier, a preamplifier and the output amplifier. In order to preserve the linearity of amplification, the power values of the preamplifier and of the output amplifier are increased by a coefficient (1+D).sup.2 in relation to to what is needed to achieve linear amplification of the first independent signal alone.
While the increase in the power of the preamplifier is relatively inexpensive, this is not the case for the output amplifier when the output power values are in the range of tens or even hundreds of kilowatts.